Population studies of a commercially fished bivalve mercenaria mercenaria (L), in Southampton Water

Al-Sayed, Hashim Ahmed Yousif (1988) Population studies of a commercially fished bivalve mercenaria mercenaria (L), in Southampton Water. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

The hard shell clam Mercenaria mercenaria (L.) is a long-lived animal tolerating wide environmental variations. The animal is an important constituent of the macrobenthic fauna in Southampton Water. A study was made of various aspects of the population in relation to absence of cooling water discharge caused by closure of Marchwood power station, increased fishing activities and the frequent failure of settlement in recent years. The littoral population at 12 sites was sampled in two successive winters. The highest density of clams was at Netley and the lowest at Royal Pier, with actively growing and reproducing populations extending to Solent Breezes and Lee-on-Solent in the eastern Solent. The total stock estimated for littoral sites above Hamble Spit has shown a reduction of about 50% since 1970. The population at present is dominated by large and old individuals originating from settlements in 1971, 1972, 1973 and 1976. These year classes and a recent settlement in 1984 seem to coincide with low river flow during August, September and October of each year. The annual production of littoral Mercenaria populations at both Marchwood and Lee-on-Solent approximated to 3gm^-2yr.^-1. Although Mercenaria dominates the biomass of the benthic fauna, energetically its role in the ecosystem has been diminished in recent years as a result of change in the population structure towards larger/older individuals. Growth in Mercenaria was analyzed according to the Logistic, the Gompertz and the Monomolecular models. The Gompertz provides an accurate model of ontogenic growth in Mercenaria. Highest predictive growth rate in shell length was found in littoral population at Solent Breezes, Hamble Spit, Weston and Netley. Maximum growth rates in shell length were achieved during the 2nd and 3rd years, whereas maximum growth rates in flesh weight were recorded during the 4th and 5th years, indicating that during the early stages and before gonad maturation a greater proportion of energy is directed to formation of shell rather than in flesh. Monthly gamete counts at Marchwood and Lee-on-Solent permitted assessment of the reproductive cycle, and the consequences of gamete production on variation of growth at both sites were discussed. It appears that at Marchwood a greater proportion of available energy is devoted to gamete production and less to somatic growth than at Lee-on-Solent. The gamete counting also revealed that the peak of spawning occurred at the end of summer and during autumn, this result was supporting data obtained by weekly monitoring of larvae. Results suggest that Mercenaria is capable of spawning at 16^oC, which is a lower temperature than that reported in earlier works, indicating a physiological adaptation to local conditions. Growth, survival and environmental factors affecting abundance of planktonic larvae over an 8 days period were studied. It appears that larvae remain in the plankton for about 15 days, at summer temperature. The 12 hours survey of vertical distribution provides evidence that larvae are retained within Southampton Water through behavioural regulation during different stages of the tide. The distribution, seasonal growth and survival of spat throughout the estuary, were investigated. First settlement of the spat in 1984 took place in the latter part of August. The population is seen as stock in decline largely resulting from overfishing and failure of spat settlement in recent years.